Objective To propose a novel mechanical method and index to in-vivo characterize the health status of cancellous bone during the percutaneous kyphoplasty, and validate its feasibility and consistency. Methods According to the theory of elasticity, the expression and physical significance of the mechanical index K were given. Then using clinical images of the lumbar spine L4, three-dimensional finite element simulations were conducted to verify the validity of the theoretical results, as well as the consistency of the methodology and the indexes were verified for studies of different balloon shapes and puncture routes. Results The internal pressure of the balloon linearly varied with the injected fluid volume. The mechanical index K was closely related to the bone shear modulus and could well reflect the health status of cancellous bones. The balloon shape had a trivial influence on the K results, and the relative difference between the cylindrical and ellipsoidal shapes was less than 2%. The influence of surgical access route on the K results was also very small, and the relative difference between the routes by vertebral pedicle and by lateral margin of vertebral pedicle was less than 0.5%. Conclusions The in-vivo mechanical method and the mechanical index K can characterize the bone health of patients with good consistency. This study has a great significance for providing guidelines of the optimization of PKP operation plan and postoperative rehabilitation, collecting in vivo data of bone mechanical properties, and improving the diagnosis and treatment of osteoporosis in clinic.